July 18, 2014

A collision of one universe with another would leave what Perimeter Institute's Matthew Johnson, calls “a disk on the sky” – a circular bruise in the cosmic microwave background. That the search for such a disk has so far come up empty makes certain collision-filled models less likely. The Perimeter team is at work figuring out what other kinds of evidence a collision might leave behind. It’s the first time that anyone has produced a direct quantitative set of predictions for the observable signatures of bubble universe collisions. And though none of those signatures has so far been found, some of them are possible to look for.

So to start, forget the Big Bang; in the beginning was the vacuum. The vacuum simmered with energy (variously called dark energy, vacuum energy, the inflation field, or the Higgs field). Like water in a pot, this high energy began to evaporate – bubbles formed.

Each bubble contained another vacuum, whose energy was lower, but still not nothing. This energy drove the bubbles to expand. Inevitably, some bubbles bumped into each other. It’s possible some produced secondary bubbles. Maybe the bubbles were rare and far apart; maybe they were packed close as foam.

But here’s the thing: each of these bubbles was a universe. In this picture, our universe is one bubble in a frothy sea of bubble universes. It’s not a bad story. It is, as scientists say, physically motivated – not just made up, but rather arising from what we think we know about cosmic inflation.

Cosmic inflation isn’t universally accepted – most cyclical models of the universe reject the idea. Nevertheless, inflation is a leading theory of the universe’s very early development, and there is some observational evidence to support it.

Inflation holds that in the instant after the big bang, the universe expanded rapidly – so rapidly that an area of space once a nanometer square ended up more than a quarter-billion light years across in just a trillionth of a trillionth of a trillionth of a second. It’s an amazing idea, but it would explain some otherwise puzzling astrophysical observations.

Inflation is thought to have been driven by an inflation field – which is vacuum energy by another name. Once you postulate that the inflation field exists, it’s hard to avoid an “in the beginning was the vacuum” kind of story. This is where the theory of inflation becomes controversial – when it starts to postulate multiple universes.

Proponents of the multiverse theory argue that it’s the next logical step in the inflation story. Detractors argue that it is not physics, but metaphysics – that it is not science because it cannot be tested. After all, physics lives or dies by data that can be gathered and predictions that can be checked.

That’s where Perimeter's Johnson comes in. Working with a small team that also includes Perimeter Faculty member Luis Lehner, Johnson is working to bring the multiverse hypothesis firmly into the realm of testable science. Johnson is a CalTech educated physicist who focuses on cosmology, field theory, string theory, and gravitation. He also designs data analysis algorithms to confront fundamental theory with observations of the Cosmic Microwave Background (CMB) radiation.

“That’s what this research program is all about,” he says. “We’re trying to find out what the testable predictions of this picture would be, and then going out and looking for them.”

Specifically, Johnson has been considering the rare cases in which our bubble universe might collide with another bubble universe. He lays out the steps: “We simulate the whole universe. We start with a multiverse that has two bubbles in it, we collide the bubbles on a computer to figure out what happens, and then we stick a virtual observer in various places and ask what that observer would see from there.”

“Simulating the universe is easy,” says Johnson. Simulations, he explains, are not accounting for every atom, every star, or every galaxy – in fact, they account for none of them. “We’re simulating things only on the largest scales,” he says. “All I need is gravity and the stuff that makes these bubbles up. We’re now at the point where if you have a favourite model of the multiverse, I can stick it on a computer and tell you what you should see.”

That’s a small step for a computer simulation program, but a giant leap for the field of multiverse cosmology. By producing testable predictions, the multiverse model has crossed the line between appealing story and real science.

In fact, Johnson says, the program has reached the point where it can rule out certain models of the multiverse: “We’re now able to say that some models predict something that we should be able to see, and since we don’t in fact see it, we can rule those models out.”

The real significance of this work is as a proof of principle: it shows that the multiverse can be testable. In other words, if we are living in a bubble universe, we might actually be able to tell.

Comments

I don't know if anyone has yet proposed this point before

If we forget about the dimensions 1-3 at the moment of big bang and inflation...

the big bang and inflation happen "everywhere" instantly. It is not as if the universe expanded at all. The inflation really means only that the dimensions 1-3 just shaped at that time and that gave the impression of expansion. In a way the giant explosion of big bang happened simultaneously everywhere locally and remotely across huge distances.

There is no reason to perceive the universe from our 3-4 dimensional point of view. It would be better to perceive it multi-dimensionally so that the first 3 aren't the prerequisite for the following ones.

Are all universes perfectly spherical? Would a ‘dent’ outline from another universe always be perfectly circular in circumference, as there might be if two perfectly symmetric universes gently ‘bumped' head-on in a perpendicular mode? Or might there be dent asymmetries ( non-circular, ‘drag’ shapes) introduced at the contact 'point' from oblique contact, or from velocity differences and/or field interactions, or from non-flat universes, etc..
These are optional collision mechanisms that might lead to a family of shapes in the Cosmic Microwave Background (CMB). But these collision modes will not have equal probability - for instance, a gentle, perpendicular bumping by a perfectly flat and dimensionally symmetric sister-universe seems a stretch, or in words, this collision mode would appear to have a comparatively null group population.

It should be possible to do a background survey and analysis of shapes and intensities of CMB anomalies. A follow-on statistical work up might yield patterns to be compared to expectations of these shapes and their group intensities (frequency of occurrence and energy transfer effectiveness) - there will be varying degrees of freedom with different shapes owing to redundancies of collision modes and different mode angles will transfer varying but characteristic energy. Might this analysis show that the CMB picture is rife with 'dents' from contact with sister universes?

Time: is the point of contact confined time-wise or spread out over billions of years as the universes make contact. Just like an analysis of the contact mechanism (grazing, direct rebound or head-on absorption or mutual destruction) might reveal shape and energy transfer variations, impact time duration should also have patterns and energy transfer efficiencies. These optional collision mechanisms and life-times that might add to and influence shape-families and corresponding intensities in the Cosmic Microwave Background (CMB) pattern/energy distributions.

Might these analyses show the CMB picture is rife with 'dents' from contact with sister universes?

Further, the intrusion into our 3-D universe would be analogous to a finger poked into a 2-D ‘Flatland’. In a 3-D universe (or higher) the 3-D shape of the “dent" resulting from a ‘poke’ might be an ovoid with tapering, perhaps even sharp, edges. Or maybe it would be a polyp shape. But, we would only see an outline - and rarely a circular outline as a perfectly head-on, "Normal", contact (a collision) will be rare. (One bit of evidence a "Normal/Orthogonal" collison would be rare is that mode represents possible mutual destruction of both universes or at least major disruption that might end conditions for starting and sustaining life....and this article and its readers would not exist.)

At very large interstellar distances, using conventional telescopes, the inward dent from a grazing sister-universe would appear flat but an interferometer 'telescope’ of maybe inter-planetary length might reveal 3-D features of the intrusion.

I am sorry that still, has not been studied and has not been understood the theory “Pointal Charges”, described in the book “From the inside of quarks and up to beyond the Universe”. If we had understood the theory of “Pointal Charges”, this article would have much different and much more essential meaning.

who says a collision with another universe would leave a bruise? I don`t believe this. I'm pretty sure there would be no marking of another expanding universe would interfere with ours, except of course of a huge number of galaxies coming toward us at the limit orison of our telescopes in a certain spot of the sky. The chance to see that is slice, because when we look so far, we see back in history billions of years. If a collision would happen now, "we" would see this phenomena after 10 billion years.

@ pasi - their are no straight lines in nature, so I don't believe a universe would be square or triangle, i.e. the corner of a universe hitting our round bubble... take a look at Co2 bubbles in a soda or beer and look how they touch each other

The only certain disk-like shape we can observe is the galactic disks and even these aren´t flat, but slightly sperical in shape.

Galaxies orbits in clusters and superclusters and even they orbits larger centers and so on in the vast voids of the observable universe. What is beyound the farthest horizons we´ll never know.

The only thing we can discuss is the local observable patterns and formations and deduct from these how all the rest may look like.

In this matter, all fundamental forces and all known natural dynamics are needed in order to describe how things works on all levels.

BTW: "The Search for an Alien Universe"- headline remind me of "The search for Alien Aliens" and almost for the same reasons:

Because we have forgotten the intuitive language of nature and the ancient symbolic language of Mytho-Cosmology, we now have alienated us selves in such degree that everything become more and more strange - included the language of modern science itself.